Take the random-access index i and adjust it so that it points to the beginning of a code point. The input index points to any code unit of a code point and is moved to point to the first code unit of the same code point. i is never incremented. In other words, if i points to a trail surrogate that is preceded by a matching lead surrogate, then i is decremented. Otherwise it is not modified. This can be used to start an iteration with UTF_NEXT_CHAR() from a random index. Same as UTF16_SET_CHAR_START.